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  Nano-crystal austenitic steel bulk material having ultra-hardness and toughness and excellent corrosion resistance, and method for production thereofUSPTO Application #: 20060193742Title: Nano-crystal austenitic steel bulk material having ultra-hardness and toughness and excellent corrosion resistance, and method for production thereof Abstract: The invention provides a super hard and tough, nano-crystal austenite steel bulk material having an improved corrosion resistance, and its preparation process. The austenite steel bulk material comprises an aggregate of austenite nano-crystal grains containing 0.1 to 2.0% (by mass) of a solid solution type nitrogen, wherein an oxide, nitride, carbide or the like of a metal or semimetal exists as a crystal grain growth inhibitor between and/or in said nano-crystal grains. For preparation, fine powders of austenite steel-forming components, i.e., iron and chromium, nickel, manganese, carbon or the like are mixed with a substance that becomes a nitrogen source. Mechanical alloying (MA) is applied to the mixture, thereby preparing nano-crystal austenite steel powders having a high nitrogen concentration. Finally, the austenite steel powders are consolidated by sintering by means of spark plasma sintering, rolling or the like. (end of abstract)
Agent: Dellett & Walters - Portland, OR, US Inventors: Harumatsu Miura, Nobuaki Miyao, Hidenori Ogawa, Kazuo Oda, Munehide Katsumura, Masaru Mizutani USPTO Applicaton #: 20060193742 - Class: 419052000 (USPTO) Related Patent Categories: Powder Metallurgy Processes, Powder Metallurgy Processes With Heating Or Sintering, Heat And Pressure Simultaneously To Effect Sintering, Heating By Electrical Means Other Than Radiant Energy The Patent Description & Claims data below is from USPTO Patent Application 20060193742. Brief Patent Description - Full Patent Description - Patent Application Claims
ART FIELD [0001] The present invention relates generally to a metal material, and more particularly to a super hard and tough nano-crystal austenite steel bulk material with an improved corrosion resistance, and its preparation process. BACKGROUND OF THE INVENTION [0002] As the Hall-Petch relationship teaches, metal material strength increases with decreasing crystal grain diameter D, and such strength dependency on grain diameter holds even at or near D=50 to 100 nm that means nano-size level crystal grains. Thus, reducing crystal grain diameters down to the ultra-fine, nano-size levels now becomes one of the most important means ever for the reinforcement of metal materials. Some technical journals suggest that reducing D down to ultra-fine sizes of as fine as a few nm causes superplasticity to come out. [0003] There are also some reports that regarding magnetic elements such as iron, cobalt and nickel, in nano-order grain ranges coercive force decreases and soft magnetism improves with decreasing D, which are not found when the crystal grain diameter D is in micron-order ranges. [0004] However, the crystal grain diameter D of most metal materials produced by melting are usually on the order of a few microns to a few tens of microns, and D can hardly be reduced down to the nano-order even by post-treatments. Even with controlled rolling that is an important micro-processing of steel crystal grains, for instance, the lowest possible limit to grain diameters is of the order of at most 4 to 5 .mu.m. In other words, with such ordinary processes it is impossible to obtain materials whose grain diameters are reduced down to the nano-size level. [0005] For instance, intermetallic compounds such as Ni.sub.3Al, Co.sub.3Ti, Ni.sub.3(Si, Ti) and TiAl that provide useful heat-resistant materials and super hard materials, and oxide- and non-oxide based ceramic materials such as Al.sub.2O.sub.3, ZrO.sub.2, TiC, Cr.sub.3C.sub.2, TiN and TiB.sub.2 are all generally less susceptible to plastic processing at normal temperature because of being fragile, and forming processes using super plasticity in relatively high temperature regions become very important. [0006] For the development of superplasticity, however, it is required to reduce their crystal grain diameters down to the nano-size level or an nano-order close thereto. Never until now are there any ultra-fine powders sufficient to meet such forming processes available. [0007] As nitrogen (N) in an amount of, e.g., about 0.9% (by mass) is added to a chromium-nickel type stainless steel having a composition equivalent to that of SUS 304 that is typical austenite stainless steel, the resulting stainless steel having a high nitrogen concentration increases in offset yield strength (yield strength) to about three times as high as that of SUS 304 stainless steel, with no decrease in fracture toughness yet with much more improvements in corrosion resistance in general and pitting corrosion resistance in particular and much more reductions in sensitivity to stress corrosion cracking. Moreover, nitrogen, because of being an extremely strong austenite-stabilization element, is not only capable of superseding expensive nickel with no damage to the above strength properties and corrosion resistance, but also has superior properties such as the effect on holding back process-inducing martensitic transformation under intensive cold processing conditions. [0008] Such effects of N are also true for chromium-manganese type austenite steels. From such considerations, chromium-nickel and chromium-manganese type austenite steels having a high nitrogen concentration have recently attracted considerable attentions as the coming generation of promising new materials. [0009] So far, high-N austenite steels having nitrogen in an amount of up to about 0.1 to 2% (by mass) have been manufactured by melting solidification processes usually in nitrogenous atmospheres, high-temperature solid diffusion sintering processes in nitrogen gas atmospheres, etc. With those processes, however, it is required that the higher the concentration of nitrogen in the end steel, the higher the pressure of nitrogen gas in the atmosphere, offering problems in connection with high-temperature, high-pressure operations and work safety. [0010] Referring here to generally available steel materials inclusive of austenite steel, the finer the crystal grains, the ever higher the effect on strength (hardness) becomes, as is the case with other metals, and high-N austenite steel, too, is now intensively studied for much finer crystal grain diameters. However, it is still very difficult to reduce crystal grains down to the nano-size level; any satisfactory ultra-fine crystal grain material is not achievable as yet, although some high-N austenite steels having a crystal grain structure of the order of a few tens of .mu.m are somehow obtainable. [0011] But then, in high-manganese austenite that attracts great attention as a steel species that could have a dominant role in the coming generation of large-scale technologies (peripheral technologies in linear motor cars, superconduction applied systems, etc.), too, any material having a crystal grain structure of the nano-order is not available as yet, as is the case with the chromium-nickel, and chromium-manganese type austenite steels. DISCLOSURE OF THE INVENTION [0012] The present invention has for its objection the provision of satisfactory solutions to the above problems. [0013] Basically, the present invention makes use of mechanical milling (MM) or mechanical alloying (MA) of a powder mixture of powders of an elementary single metal and powders of other metal additives or the like. The resulting nano-crystal fine powders are cosolidated by forming-by-sintering, thereby providing a bulk material, composed of an aggregate of grains of nano-size levels, and having strength (high strength) or hardness (super hardness) close to the finest possible limit. Furthermore, crystal grains of magnetic elements such as iron, cobalt and nickel are reduced down to nano-size levels so as to provide a novel material showing much better soft magnetism. [0014] The present invention also provides a novel process for preparing a non-magnetic, high-nitrogen nano-crystal austenite steel material having super hardness and toughness with an improved corrosion resistance (pitting-corrosion resistance) by applying mechanical alloying (MA) to an elementary powder mixture of iron and chromium, nickel, manganese, carbon or the like with a nitrogen source substance such as iron nitride, using a ball mill or the like and then applying forming-by-sintering to the resultant nano-crystal austenite steel fine powders, thereby obtaining a nano-crystal austenite steel bulk material containing a solid-solution type nitrogen in an amount of preferably 0.1 to 2.0% (by mass), more preferably 0.3 to 1.0% (by mass), and even more preferably 0.4 to 0.9% (by mass). [0015] Furthermore, the present invention provides a high-manganese austenite steel having a nano-order crystal structure through the application of mechanical alloying and forming-by-sintering similar to that mentioned above. [0016] Thus, the present invention is concerned with austenite steel bulk materials constructed as recited below, and their preparation processes and uses. [0017] (1) A super hard and tough austenite steel bulk material with an improved corrosion resistance, comprising an aggregate of austenite nano-crystal grains containing a solid-solution type nitrogen in an amount of 0.1 to 2.0% (by mass), characterized in that a metal oxide or a semimetal oxide exists as a crystal grain growth inhibitor between or in said nano-crystal grains, or between and in said nano-crystal grains. [0018] (2) A super hard and tough austenite steel bulk material with an improved corrosion resistance, comprising an aggregate of austenite nano-crystal grains containing a solid-solution type nitrogen in an amount of 0.1 to 2.0% (by mass), characterized in that a metal nitride or a semimetal nitride exists as a crystal grain growth inhibitor between or in said nano-crystal grains, or between and in said nano-crystal grains. [0019] (3) A super hard and tough austenite steel bulk material with an improved corrosion resistance, comprising an aggregate of austenite nano-crystal grains containing a solid-solution type nitrogen in an amount of 0.1 to 2.0% (by mass), characterized in that a metal carbide or a semimetal carbide exists as a crystal grain growth inhibitor between or in said nano-crystal grains, or between and in said nano-crystal grains. [0020] (4) A super hard and tough austenite steel bulk material with an improved corrosion resistance, comprising an aggregate of austenite nano-crystal grains containing a solid-solution type nitrogen in an amount of 0.1 to 2.0% (by mass), characterized in that a metal silicide or a semimetal silicide exists as a crystal grain growth inhibitor between or in said nano-crystal grains, or between and in said nano-crystal grains. [0021] (5) A super hard and tough austenite steel bulk material with an improved corrosion resistance, comprising an aggregate of austenite nano-crystal grains containing a solid-solution type nitrogen in an amount of 0.1 to 2.0% (by mass), characterized in that a metal boride or a semimetal boride exists as a crystal grain growth inhibitor between or in said nano-crystal grains, or between and in said nano-crystal grains. Continue reading... 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